The present invention is directed at a method for passivating adverse catalytic effects of metal contaminated cracking catalyst. More specifically, the present invention is directed at a method for providing improved passivation conditions and control in fluidized catalytic cracking units.
In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks, vanadium, nickel and/or iron present in the feedstock becomes deposited on the cracking catalyst promoting excessive hydrogen and coke makes. These metal contaminants are not removed by conventional catalyst regeneration operations during which coke deposits on the catalyst are converted to CO and CO.sub.2.
As used hereinafter, the term "passivation" is defined as a method for decreasing the detrimental catalytic effects of metal contaminants such as nickel, vanadium and iron which become deposited on catalyst. U.S. Pat. Nos. 3,711,422; 4,025,545; 4,031,002; 4,111,845; 4,141,858; 4,148,712; 4,148,714; and 4,166,806 are all directed to the contacting of the cracking catalyst with antimony compounds to passivate the catalytic activity of the iron, nickel and vanadium contaminants deposited on the catalyst. However, antimony compounds alone may not passivate the metal contaminants to sufficiently low levels, particularly where the metal contaminant concentration on the catalyst is relatively high.
Cimbalo, Foster and Wachtel, in an article entitled "Deposited Metals Poison FCC Catalyst", published at pages 112-122 of the May 15, 1972 issue of Oil and Gas Journal, disclose that the catalytic activity of metal contaminants decrease with repeated oxidation and reduction cycles.
U.S. Pat. No. 2,575,258 discloses the use of a reducing gas to decrease the adverse catalytic effects of ferrous contaminants in a cracking system. Reducing gas at an elevated temperature may be introduced through a plurality of points into the transfer line between the regeneration zone and the reaction zone to decrease adverse catalytic effects of the iron contaminant. One embodiment discloses the use of a valve in the transfer line between the regeneration and reaction zones to direct regenerated catalyst from the transfer line through a separate passivation zone.
U.S. Pat. Nos. 4,280,895 and 4,280,896 disclose a method for passivating metal contaminated cracking catalyst by passing the cracking catalyst through a passivation zone having a reducing atmosphere maintained at an elevated temperature for a period of time ranging between 30 seconds and 30 minutes, typically from about 2 to about 5 minutes.
U.S. Pat. No. 4,298,459 describes a process for cracking a metals containing feedstock where the cracking catalyst is subjected to alternate exposures of up to 30 minutes in an oxidizing zone and in a reducing zone maintained at an elevated temperature to thereby reduce the hydrogen and coke makes. U.S. Pat. Nos. 4,268,416; 4,361,496; and 4,364,848; and PCT Patent Publication No. WO/04063 all describe methods for passivating cracking catalyst in which metal contaminated cracking catalyst is contacted with a reducing gas at elevated temperatures to passivate the catalyst.
European Patent Publication No. 52,356 discloses the use of a reducing gas passivation zone having a residence time of about 3 seconds to about 2 hours for passivating the adverse effects of metal contaminants present on cracking catalyst.
U.S. Pat. Nos. 4,372,840 and 4,372,841 also disclose the use of a high temperature reducing atmosphere for metals passivation. These patents further disclose that addition of a hydrogen donor material to the reaction zone reduces the hydrogen and coke makes.
U.S. Pat. No. 3,857,794 discloses the addition of CO to the regeneration zone to decrease or eliminate afterburning.
U.S. Pat. No. 3,408,286 discloses the use of a valve in the transfer line between a reaction zone and a regeneration zone. This patent also discloses addition of a feed slipstream upstream of the valve to displace combustion gases from the interstices of the catalyst.
U.S. Pat. No. 4,345,992 discloses the addition of a reducing gas between the disengaging zone and the reaction zone to passivate metal contaminants on cracking catalyst. This patent also discloses the use of a gaseous seal upstream of the reducing gas addition point to minimize the amount of reducing gas flowing into the disengaging zone.
It would be advantageous to utilize the standpipe and/or U-bend of the cracking system between the regeneration zone and the cracking zone as a passivation zone to passivate cracking catalyst, if this passivation zone provides a sufficient residence time and if the use of the standpipe and/or U-bend as a transfer line does not cause any operational problems.
It is desirable to provide a process in which the catalyst may be passivated without the installation of a separate passivation vessel.
It also is desirable to provide a process in which the residence time of the catalyst in the passivation zone is controllable.
It also is desirable to provide a process in which the flow of reducing gas into the regeneration zone is minimized.
It also is desirable to provide a process in which the flow of reducing gas may be regulated during process upsets.
The present invention is directed at a method for regulating the residence time of regenerated catalyst in a transfer line disposed between the reaction and regeneration zones. The present invention also is directed at the monitoring of key process variables in the cracking system and regulating the rate of addition of reducing gas in response to these variables. Passivation promoters or rate enhancers such as antimony, tin, bismuth, maganese, cadmium, germanium, indium, tellurium and zinc may be added to increase the rate of passivation.